In recent years, as electronic technology progresses, reduction in size and weight of electronic devices has been rapidly progressing. Particularly, because of the recent spread and the increase in functionality of portable electronic devices, such as cell phones and notebook-sized personal computers, the development of a small and lightweight battery having a high energy density has been strongly desired as a portable power source to be used for those portable electronic devices.
Lithium ion secondary batteries, which are nonaqueous-electrolyte secondary batteries, are small in size and have a high energy, and therefore, have been already applied as a power source for portable electronic devices. Furthermore, lithium ion secondary batteries find applications other than those limited applications, and, there have been also carried out the research and development of lithium ion secondary batteries to make use of the batteries as a large-sized power source for hybrid vehicles, electric vehicles, and the like.
As a positive electrode active material for lithium ion secondary batteries, lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized, has been made use of, but, as a raw material of lithium-cobalt composite oxide, a cobalt compound, which is rare and expensive to be produced, has been used, and therefore, an increase in the cost of the positive electrode active material has been caused. A reduction in the cost of the positive electrode active material and the resulting realization of manufacturing of more-inexpensive lithium ion secondary batteries can be said to have great industrial significance because such cost reduction and realization allow the cost of currently-widespread portable electronic devices to be reduced and lithium ion secondary batteries to be loaded into future large-sized power sources.
Examples of other positive electrode materials applicable as a positive electrode active material for lithium ion secondary batteries include lithium-manganese composite oxide (LiMn2O4), in which manganese, more inexpensive than cobalt, is contained; and lithium-nickel composite oxide (LiNiO2), in which nickel, more inexpensive than cobalt, is contained.
Lithium-manganese composite oxide is not only made of an inexpensive raw material, but also has high thermal stability, particularly, has high safety from ignition and the like, and therefore, it can be said that lithium-manganese composite oxide is an effective alternative material for lithium-cobalt composite oxide.
However, lithium-manganese composite oxide has only approximately half the theoretical capacity of lithium-cobalt composite oxide, and hence, has the disadvantage of difficulty in satisfying yearly-increasing demands for a high-capacity lithium ion secondary battery. Furthermore, lithium-manganese composite oxide has the disadvantages of intense self-discharge at not less than 45° C. and the resulting decrease in charge-and-discharge life.
On the other hand, lithium-nickel composite oxide has the advantage of having a higher capacity than that of lithium-cobalt composite oxide, which has been currently a mainstream, and has the advantages that nickel, a raw material, is more inexpensive and more stably available than cobalt, and therefore, lithium-nickel composite oxide has been expected as a next-generation positive electrode material, and the research and development of lithium-nickel composite oxide have been actively carried out.
However, lithium-nickel composite oxide has a problem that, in the case where a lithium ion secondary battery is produced using, as a positive electrode active material, a lithium-nickel composite oxide which is constituted not by substituting nickel with another element, but by using only purely nickel, the lithium-nickel composite oxide has poorer cycle characteristics than lithium-cobalt composite oxide. The reason for this is considered to be that the crystal structure of lithium-nickel composite oxide changes from hexagonal to monoclinic with the desorption of lithium, and furthermore, changes to hexagonal again, and this crystal structure change is poor in reversibility, and therefore, as a charge-and-discharge reaction is repeated, sites for insertion and desorption of Li are lost little by little.
To solve this problem, the substitution of cobalt for a part of nickel has been proposed (for example, refer to Patent documents 1 to 3). The substitution with cobalt prevents the phase transition of the crystal structure caused by the desorption of lithium, and a larger amount of substitution with cobalt allows a crystal phase to be more stable, whereby cycle characteristics are improved.
An object of the addition of cobalt is to stabilize a crystal phase by substituting cobalt for nickel present in a crystal structure, and therefore, cobalt and nickel need to be uniformly mixed at the atomic level. Effective is a process in which a hydroxide produced by continuously coprecipitating a nickel source and a cobalt source is used as a raw material of a positive electrode active material which realizes the object.
For example, Patent document 4 discloses that the control of the particle shape, the particle diameter, the specific surface area, the tap density, the pore space volume, and the pore filling factor of a nickel-cobalt coprecipitation hydroxide prevents cycle deterioration and achieves a battery having excellent charge-and-discharge characteristics, and, actually, such control has achieved certain characteristics.
However, a conventional process for the foregoing synthesis of a hydroxide by continuously coprecipitating a nickel source and a cobalt source has a problem that it is difficult to manufacture a nickel-cobalt composite hydroxide having a uniform particle diameter. This is because, in the manufacturing of nickel-cobalt composite hydroxide by a continuous crystallization reaction, nucleation by precipitation and a growth reaction of each of particles simultaneously proceed.
In the case where a positive electrode active material having a wide distribution of particle size is used, non-uniform voltages are applied to particles in an electrode, and accordingly, a repetition of charge and discharge causes the selective deterioration of fine particles and a decrease in capacity. Therefore, a positive electrode active material is required to comprise particles having a uniform and appropriate particle diameter.
As for achieving a uniform particle diameter, for example, Patent document 5 proposes a process for manufacturing nickel-cobalt-manganese composite hydroxide particles, the process including: a nucleation step in which nucleation is performed by controlling a solution for nucleation so as to achieve a pH value of from 12.0 to 14.0, the pH value being measured at a reference liquid temperature of 25° C.; and a particle growth step in which a solution for particle growth containing nuclei formed in the nucleation step is controlled so as to have a pH value of from 10.5 to 12.0 to grow the foregoing nuclei, the pH value being measured at a reference liquid temperature of 25° C.
Patent document 5 describes that the separate implementation of the nucleation and the particle growth allows the achievement of nickel-cobalt-manganese composite hydroxide particles having a small particle diameter and a uniform distribution of particle size, and a positive electrode active material manufactured using these particles as a raw material is excellent in particle diameter uniformity.
However, in Patent document 5, an index indicating the degree of a particle size distribution, namely, [(d90−d10)/average particle diameter] is not more than 0.55, and hence, in the case of a biased distribution of particle size, it cannot necessarily be said that fine particles or coarse particles are prevented from being mixed in. Furthermore, the foregoing manufacturing process is limited to a batch type, and hence, the process cannot be said to have high manufacturing efficiency.
Therefore, there has been demanded a positive electrode active material in which fine particles and coarse particles are further prevented from being mixed in, which has high particle-diameter-uniformity, and which is capable of being manufactured with high productivity.